The present invention relates generally to a driving method for driving luminous elements to emit light, and more particularly to a driving method for driving current injection type luminous elements having a capacitance component, such as an organic electroluminescent device (EL).
A conventional organic EL driving method is disclosed in U.S. Pat. No. 5,844,368 (JP-A-9-232074). This driving system, which is shown in FIG. 10, is a matrix driving method in which anode lines A1 through Am and cathode lines B1 through Bn are disposed in a matrix (grid). One of the anode lines and cathode lines is sequentially selected and scanned at a fixed time interval, and another line is driven by a drive source, that is, current line 11 through 1m, in synchronism with this scan line to cause a luminous element at a desired intersection of anode and cathode lines to emit light.
Scanning switches 21 through 2n for selecting either the supply voltage (VCC) or ground potential (O V) are connected to the cathode lines B1 through Bn for sequentially scanning the cathode lines. The ground potential (0V) is sequentially applied to the cathode lines B1 through Bn by scanning while switching switches 21 through 2n sequentially to the ground terminal side at fixed time intervals. Drive switches 31 through 3m for selecting the current source 11 through 1m, that is, the drive source, or the ground potential (0V), are connected to anode lines A1 through Am. Drive current is supplied to a luminous element at a desired anode-cathode intersection by connecting current source 11 through 1m to anode line A1 through Am by switching the drive switches 31 through 3m on and off in synchronism with the scanning switches.
Driving the luminous elements E1.2 and E1.3 to emit light is described by way of example below. When scanning switch 21 is switched to the ground side and a ground potential is applied to a first cathode line B1 as shown in FIG. 10, luminous elements E1.2 and E1.3 can be made to emit light by switching drive switches 32 and 33 to the current source side and connecting current sources 12 and 13 to anode lines A2 and A3. The luminous elements are controlled so that the luminous element at an arbitrary position emits light and so that the luminous elements appear to emit light concurrently by quickly repeating such scan and drive.
In addition, the reverse bias voltage VCC, which is equal to the source voltage potential, is applied to each of the cathode lines B2 through Bn. The reverse bias voltage VCC is not applied to the cathode line B1 being scanned in order to prevent erroneous emission.
Each of the luminous elements E1.1 through En.m connected at each intersection may be represented by a luminous element E having a diode characteristic and a parasitic capacitor C connected in parallel, as shown by the equivalent circuit in FIG. 11. However, this driving method has the following problems due to the parasitic capacitor C within the equivalent circuit.
FIGS. 12A and 12B show each of the luminous elements E1,1 through En,1 using only the parasitic capacitors C by excerpting the part of the luminous elements E1.1 through En.1 connected to the anode line A1 in FIG. 10. FIGS. 12C and 12D show each of the luminous elements E1.2 through En.2 using only the parasitic capacitors C by excerpting the part of the luminous elements E1.2 through En.2 connected to the anode line A2 in FIG. 10.
When the cathode line B1 is scanned and the anode line A1 is not driven, parasitic capacitor C1.1 of the luminous element E1.1 connected to the cathode line B1 currently being scanned is not charged. However, other parasitic capacitors C2.1 through Cn.1 of luminous elements E2.1 through En.1 are charged in a direction shown in FIG. 12A.
It is assumed that the scanning position is shifted from the cathode line B1 to the next cathode line B2 and the anode lines A1 and A2 are driven in order to cause the luminous elements E2.1 and E2.2 to emit light. The state of the circuit when anode line A1 is driven to drive luminous element E2.1 to emit light is shown in FIG. 12B, and the state of the circuit when anode line A2 is driven to cause luminous element E2.2 to emit light is shown in FIG. 12D.
When luminous element E2.1 is driven to emit light, not only is the parasitic capacitor C2.1 of the luminous element E2.1 charged, but the parasitic capacitors C3.1 through Cn.1 of the luminous elements E3.1 through En.1 connected to the other cathode lines B3 through Bn also are charged because currents flow into the capacitors in the direction as indicated by arrows. On the other hand, when luminous element E2.2 is driven to emit light, only parasitic capacitor C2.2 of luminous element E2.2 is charged as shown in FIG. 12D. It will be noted because the charge causing luminous elements E2.1 and E2.2 to emit light differs greatly, the time needed for the end-to-end voltage of luminous elements E2.1 and E2.2 to reach the level required for the luminous elements to emit light also differs greatly. Accordingly, the brightness of luminous elements E2.1 and E2.2 differs, resulting in uneven luminance.
Another matrix driving method is disclosed in JPA-9-232073. This method, drives organic EL elements to emit light by connecting organic EL elements at the anode line and cathode line intersections of the grid. This method first resets all scanning lines to the same voltage potential when shifting to the next scanning line. This increases the build up speed from applying a voltage to emission.
This method is described next with reference to FIG. 13 through FIG. 15.
In FIG. 13, at first the scanning switch 21 is switched to 0V and the cathode line B1 is scanned. The reverse bias voltage is applied to the other cathode lines B2 through Bn via the scanning switches 22 through 2n. Further, the current sources 11 and 12 are connected to the anode lines A1 and A2 via the driving switches 31 and 32. Still further, 0V is applied to the other anode lines A3 through Am via the drive switches 33 through 3m. 
Accordingly, FIG. 13 illustrates that only the luminous elements E1.1 and E1.2 emit light because only these elements are biased in the forward direction such that driving currents flow into these elements from the current sources 11 and 12, as indicated by arrows in the figure. In the state of FIG. 13, the luminous elements indicated by a hatched capacitor are being charged, respectively, in the direction of the polarity shown in the figure. Then, the reset control shown in FIG. 14 is carried out in shifting the scan so that the luminous elements E2.1 and E2.3 emit light as shown in FIG. 15.
That is, before shifting the scan from the cathode line B1 in FIG. 13 to the cathode line B2 in FIG. 15, all of the driving switches 31 through 3m and scanning switches 21 through 2n are switched to 0V to shunt all of the anode lines A1 through Am and the cathode lines B1 through Bn to 0V, as shown in FIG. 14, thus discharging any electric charge stored or charged in each luminous element.
After discharging the electric charge stored in all of the luminous elements to zero, only the scanning switch 22, which corresponds to the cathode line B2, is switched to the side of 0V to scan the cathode line B2 as shown in FIG. 15. At the same time, drive switches 31 and 33 shunt the anode lines A1 and A3 to the current sources 11 and 13, and drive switches 32 and 34 through 3m are switched to the O V side to apply 0V to the other anode lines A2 and A4 through Am. As a result, only luminous elements E2.1 and E2.3 are biased in the forward direction in the case shown in FIG. 15. Thus, drive current flows from current sources 11 and 13 as shown by arrows so that only luminous elements E2.1 and E2.3 emit light.
Differences in the charge state (FIG. 12A and FIG. 12C) arising from the emission state when cathode line B1 is scanned are cancelled at this time, because the charge stored to all luminous elements is reset to 0V before scanning cathode line B2. As a result, the build-up to emission of luminous elements E2.1 and E2.3 becomes substantially simultaneous. Thus, uneven luminance is solved.
It should be noted here that it is desirable to apply the reverse bias voltage to the luminous elements when driving luminous elements such as organic EL elements in order to increase the service life of these elements. That is, it is desirable in the above method to apply VCC to the cathode line, apply 0V to the anode line, and apply the reverse bias (−VCC) to each luminous element at least once each frame period.
The voltages applied to each luminous element in the above system is shown in FIG. 16. In the state shown in FIG. 16 all luminous elements E1.1, E2.1, E3.1 . . . En.1 on anode line A1 are emitting light (ON), and luminous elements E1.2, E2.2, E3.2 . . . En.2 on anode line A2 are repeatedly emitting light and not emitting light (OFF). The voltages applied to luminous elements E1.1 and E1.2 are shown in FIG. 16. The difference of the voltage of anode line A1 and the voltage of cathode line B1 is applied to luminous element E1.1, and the difference of the voltage of anode line A2 and the voltage of cathode line B1 is applied to luminous element E1.2.
It is understood from the voltage applied to luminous element E1.2 that the reverse bias voltage is applied in the period when luminous element E2.2, for example, is not emitting light.
However, with respect to the voltage applied to luminous element E1.1, there is no period in which the reverse bias is applied because all luminous elements on anode line A1 are emitting light. This is not desirable with respect to luminous element service life.
Furthermore, as shown by the equivalent circuit in FIG. 11, a luminous element such as an organic EL element can be represented as a luminous element E with a diode characteristic and a parallel-connected parasitic capacitor C. The prior art driving method cancels the capacitor effect by discharging the charged capacitance of all luminous elements by connecting all cathode lines to a reset voltage when switching the scanning lines.
With the above driving method, however, the capacitance on the cathode line of the luminous elements to emit light next is also discharged, thus requiring more time to charge the luminous element driven to emit light next, and the build-up speed to emission is thus slow. Fast scanning is therefore not possible.